Oxidizing process and composition for aluminum

ABSTRACT

The invention is for a process for providing an oxide coating on aluminum which process uses chemical means only while avoiding the use of an electric current as is required in an anodizing process. Absent conventional steps such as water rinses and the like, the process comprises cleaning, including desmutting of an aluminum part to the extent necessary, and treating with an aqueous alkaline solution of a ferricyanide compound for a time sufficient to oxidize the aluminum part. The treatment with the ferricyanide solution is considered to be a chemical oxidizing step similar to the formation of an oxide film electrically in an anodizing process, but lower in cost and having other advantages. The oxidized aluminum surface is corrosion and heat resistant, acts as a base for various coating and is readily dyed to produce brilliant colors.

United States Patent [1 1 Tuttle et al.

[ *July 29, 1975 OXIDIZING PROCESS AND COMPOSITION FOR ALUMINUM [75]Inventors: Bertha S. Tuttle, I-larwichport;

Jekabs Ozolins, Holliston, both of Mass.

[73] Assignee: J. N. Tuttle, Inc., Medford, Mass.

[ Notice: The portion of the term of this patent subsequent to Oct. 16,1990, has been disclaimed.

[22] Filed: Sept. 21, 1973 [21] Appl. No.: 399,753

Related U.S. Application Data [63] Continuation-impart of Ser. No.170,668, Aug. 10,

1971, Pat. No. 3,765,952.

3,765,952 10/1973 Tuttle et al. l48/6.l

OTHER PUBLICATIONS Chem. Abstracts, 72:27579y, 2-9-70.

Primary ExaminerRalph S. Kendall Assistant ExaminerCharles R.Wolfe, Jr.Attorney, Agent, or F irmR0bert L. Goldberg [57] ABSTRACT The inventionis for a process for providing an oxide coating on aluminum whichprocess uses chemical means only while avoiding the use of an electriccurrent as is required in an anodizing process. Absent conventionalsteps such as water rinses and the like, the process comprises cleaning,including desmutting of an aluminum part to the extent necessary, andtreating with an aqueous alkaline solution of a ferricyanide compoundfor a time sufficient to oxidize the aluminum part. The treatment withthe ferricyanide solution is considered to be a chemical oxidizing stepsimilar to the formation of an oxide film electrically in an anodizingprocess, but lower in cost and having other advantages. The oxidizedaluminum surface is corrosion and heat resistant, acts as a base forvarious coating and is readily dyed to produce brilliant colors.

18 Claims, No Drawings OXIDIZING PROCESS AND COMPOSITION FOR ALUMINUMCROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of U.S. application Ser. No. 170,668 filed Aug. 10,1971, now U.S. Pat. No. 3,765,952.

BACKGROUND OF THE INVENTION 1. Introduction This invention relates tothe formation of oxide coatings on aluminum and more particularly, to aprocess and composition for oxidizing aluminum using chemical means onlyavoiding the use of an electric current.

2. Description of the Prior Art Methods for providing an integral oxidecoating on aluminum are well known in the art. These coatings arecorrosion resistant, may be dyed, and act as bases for various finishessuch as paint. The most common method for forming such a coating is anelectrochemical method called anodizing which method involves theformation of an oxide coating by passing a current through anelectrolyte such as a sulphuric acid or a chromic acid solution wherethe aluminum part to be oxidized is the anode and the tank containingthe electrolyte is the cathode. Following formation of the oxidecoating, the aluminum part may be immersed in a dye bath to impart thedesired coloration to the aluminum. A process for anodizing aluminum isdisclosed in the Metal Finishing Guidebook Directory for 1967, Metalsand Plastics Publications, Inc., Westwood, New Jersey, pages 515 to 525.

Though anodizing is one of the most widely used methods for oxidizingaluminum, it has drawbacks, one of which being the high cost of theprocess. This is due to the requirement for special equipment associatedwith passing a current through a solution. Thus, electrodes, a rectifierand special lead lined or stainless steel tanks are required. Further,after prolonged usage, the solution must be discarded due to build-up ofaluminum.

ln copending U.S. Pat. applications Ser. No. 170,668 a process foroxidizing aluminum to provide corrosion and heat resistant coatingswhich are readily dyed and receptive to various finishes is disclosed.According to the process of said application, an aluminum part iscleaned and desmutted to the extent required and contacted with anaqueous alkaline oxidizing solution of a ferricyanide compound for atime sufficient to oxidize the surface of the part. If desired, theoxidized aluminum part is then dyed with a suitable dye to effect adesired colored coating or coated with any other suitable finish such aspaint, a lacquer or the like.

The critical step in said application is considered to be the treatmentwith the aqueous alkaline ferricyanide solution which is considered tobe a chemical oxidizing step similar to the formation of the oxide filmelectrically in an anodizing process. The ferricyanide salt is used insaid solution in a concentration of at least 0.01 moles per liter andthe solution has a pH ranging between about and 13.

The process of said application overcomes the disadvantage with respectboth to electrical anodizing and to chemical treatment in accordancewith the aforesaid U.S. Pat. No. 2,796,371. With respect first toanodizing, expensive electrical equipment and special tanks are notrequired as the treatment process is completely chemical in nature. As aresult, not only may large parts be treated, but small aluminum partsmay be colored using barrel plating techniques since electricalconnections are not required. With respect to the chemical process notedabove, the process of said application is an improvement thereover, inthat the oxide coatings are harder and possess better corrosion and heatresistance properties. In addition, when dyed, the color imparted ismore uniform, brighter and substantially more desirable than thosecolors obtained with the aforesaid chemical process. Further, withsuitable treatment and replenishment, the treatment baths of saidapplication have an extremely long life, typically in excess of twoyears.

SUMMARY OF THE INVENTION The subject invention is an improvement overthat of the aforesaid U.S. Pat. application Ser. No. 170,668 in thatwith the use of the same ferricyanide oxidizing solutions, it has beenfound that the pH range of the composition can be extended below 10 andabove 13 by careful temperature regulation. Thus, by use of temperatureselevated up to close to the boiling point of the solution, the pH of thecomposition can be reduced below 10, typically down to about 7.5. By useof temperatures below room temperature, typically down to close to thefreezing point of the solution, the pH of the solution can be increasedabove 13, typically up to about 14.0.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention described hereinrelates to treatment of aluminum parts. An aluminum part is definedherein as not only a part formed from aluminum, but articles coated withaluminum such as plastic substrates coated with aluminum. The thicknessof the aluminum coating is not critical, though it should be understoodthat with extremely thin coatings, e.g. 2000 angstroms, or less, thetreating solution might attack the aluminum metal to the extent that achemically oxidized coating may not be formed.

Prior to treating an aluminum part with the oxidizing formulation ofthis invention, the part is preferably cleaned to the extent necessary.Cleaning can be by a combination of steps comprising solvent degreasing,preferably with a hydrocarbon solvent such as benzene to remove greaseand oil or by soak cleaning. Thereafter, the part may be etched with amild acid or alkaline cleaner to remove dirt, oxides and othercontaminants, but not so as to form visible surface irregularities.Finally, if necessary, the part may be treated with a conventionaldesmutter to remove any residue that may be left on the surface of thepart. Such a desmutter could comprise a mixture of dilute nitric andhydrofluoric acid. Intermediate between each of the aforesaid stepswould be a water rinse. The entire process of cleaning and desmutting isconventional in the art and not considered to be a part of thisinvention.

The next step in the process comprises formation of an oxide filmchemically, which film is considered to be similar in function to theoxide film formed electrochemically in the conventional anodizingprocess. The chemical oxidant used is an aqueous, alkaline solution of aferricyanide compound, preferably, an alkali or alkaline earthferricyanide, though ferricyanide compounds having cations other thanalkali and alkaline earth metal cations are suitable provided suchcations do not deposit on the aluminum part being treated. Theconcentration of the ferricyanide compound in solution is not critical,from 0.01 moles per liter to saturation being a satisfactory range.However, at the low concentration of 0.01 moles per liter, the processis impractical because of the long treatment time required. A preferredrange for the ferricyanide compound is from about 0.1 to 0.5 moles perliter and most preferably, from about 0.20 and 0.30 moles per liter.

The oxidizing solution is an aqueous, alkaline solution with pH adjustedto between about 7.5 to or 13 to 14.0. To obtain desired pH, a pHadjustor is preferably used such as an acid such as phosphoric acid asnecessary, hydroxide, a carbonate or a tri-basic phosphate. The alkaliand alkaline earth metal hydroxides and alkali carbonates and tri-basicphosphates are suitable, sodium and potassium carbonates, sodium andpotassium hydroxides and trisodium and tripotassium phosphates being themost preferred materials. The carbonate and tri-basic phosphate act as abuffer and are preferably used in combination, especially where thetribasic phosphate also acts as an inhibitor as will be explained inmore detail below. The amount of pH adjustor used is that amount thatgives the desired pH. Using potassium carbonate as an example, fromabout 0.01 to 0.1 moles per liter of solution provides the desired pHbelow 10 and from about 0.5 to about 0.75 moles per liter of solution ofa combination of potassium carbonate and sodium hydroxide, the sodiumhydroxide being used in the minimum amount necessary relative to thepotassium carbonate to reach the desired pH provides desired pH above13. It should be noted that potassium carbonate alone cannot be used toobtain these high pH values.

It is also desirable to include an inhibitor for aluminum in theoxidizing solution to prevent darkening of the oxide coating and toproduce brighter color if the aluminum part is subsequently contactedwith a dye during the processing sequence. A suitable inhibitor is theaforesaid tri-basic phosphate such as tri-basic sodium phosphate andtribasic potassium phosphate. Thus, the tri-basic phosphate is both aninhibitor and a pH adjustor. The amount of the tri-basic phosphate isnot critical, small amounts providing some benefit with larger amountsproviding greater benefit. In genera], the concentration may be as lowas 0.001 moles per liter to saturation or even in excess of saturation.A preferred range for the tri-basic phosphate is from 0.04 to 0.50 molesper liter.

A preferred formulation in accordance with the invention is as follows:

Potassium ferricyanide 0.1 to 0.5 moles per liter Potassium carbonate0.1 to 0.5 moles per liter Trisodium phosphate sufficient to providedesired pH Water to 1 liter The oxidizing solution of this invention isused at a temperature commensurate with the pH of the solution. Thus, ifthe pH of the solution is between 7.5 and 10, the temperature ismaintained between about 100F and the boiling point of the solution. Ifthe pH of the solution is between 13 and 14.0, the temperature of thesolution is maintained within a. range of about 2F above the freezingpoint of the solution to about 50 to 60F though temperatures up to aboutF can be used if treatment precautions are exercised. It should beunderstood that the temperature is inversely related to pH. In otherwords, as the pH is increased from 13 to 14.0, the temperature iscorrespondingly decreased and as the pH is decreased from 10 down toabout 7.5, the temperature is correspondingly increased.

The time of contact of an aluminum part with the oxidizing solution isnot critical, periods of time ranging from 1 to 60 minutes beingsuitable and from 5 to 25 minutes being typical.

In general, those aluminum alloys containing a high copper contentrequire a shorter treatment thant those with a low copper content. Itshould be understood that there is a relationship between concentrationof ingredients in the oxidizing solution, temperature and time; moreconcentrated solutions or higher temperature or a combination of the tworesulting in shorter treatment time.

Following treatment with the oxidizing solution of this invention, thealuminum part is rinsed and may be coated with a finish such as thosedisclosed in U.S. Pat. No. 2,796,371 incorporated herein by reference,or a solution of a colorant which may be either an organic dye or evenan inorganic pigment. Many of the colorants that may be used are thoseconventionally used in anodizing. Typical of such dyes are thefollowing, set forth for purposes of example only:

Aluminum Orange 3A The parts treated with the alkaline solution of theferricyanide may be colored in accordance with prior art anodizingtreatment procedures. Thus, dye concentration, treatment temperature andtime are conventional, temperatures of from room to F being appropriatewith treatment time ranging from about /2 to 20 minutes dependent ontemperature, lower temperatures requiring longer times.

Following dyeing and a water rinse, the part may be sealed if desired,using the conventional sealing step of immersion of the colored part ina solution such as nickel acetate or sodium dichromate or any otherconventional material in accordance with art-recognized procedures.

The following examples are given for the purpose of illustrating theinvention. Example 1 below is set forth to illustrate the procedure asset forth in the above noted copending application and the examples thatfollow example 1 illustrate the process of this subject invention.

EXAMPLE 1 An aluminum panel of No. 5056 alloy measuring 2 inches X 4inches X 0.016 inch is degreased by soaking for five minutes in aconventional non-etching aluminum soak cleaner made up at 60 grams perliter and maintained at 150F. The panel is then removed, water rinsed,and next immersed in a conventional mild alkaline etching cleanerconsisting of 55 grams of cleaner (Clepo No. 30R) dissolved in oneliter'of water. The cleaning bath is maintained at about l50F, the panelis removed after about one minute treatment in the bath and rinsed incold water. The clean panel is then immersed in a percent nitric acidsolution to desmut the same and provide a clean surface. A treatmenttime of A minute is used. The clean panel is rinsed with cold watertreated with a brightening solution desmutte'd, rinsed again andimmersed in a solution comprising 80 grams of potassium ferricyanide, 40grams 'of potassium carbonate, 40 grams of trisodium phosphate and waterto one liter. The pH of the solution is maintained at about 12.0, thetemperature of the solution is held at about 65F and immersion time isabout minutes. Thereafter the panel is removed and rinsed with water.The panel, having an oxide coating, is then dyed by immersion for 5minutes in a dye bath maintained at about 90F consisting of 4 grams ofChromoxane Pure Blue BA (CI. 43830) in one liter of water. The pH of thedye is adjusted to between 7.0 and 8.0. The dyed panel is rinsed withwater and sealed in a solution containing 0.5 grams of sodium dichromatedissolved in one liter of water with the pH maintained at about 5.6. Thetime of sealing is minutes and the temperature of the sealing bath ismaintained at about 80-90F. The panel is then rinsed with water, driedin air, and buffed by hand. it has a uniform bright blue coloration andhas good wear and corrosion resistance properties.

EXAMPLE 2 The procedure of example 1 is repeated except that the pH ofthe solution is reduced to about 9.5 and the concentration of thepotassium carbonate and trisodium phosphate is reduced in an amountsufficient to reduce the pH to 9.5. The temperature of the oxidizingsolution is maintained at about 190F and the immersion time is about 20minutes. All of the steps are the same. The aluminum part so treated hasa uniform bright blue coloration and good wear and corrosion resistanceproperties.

EXAMPLE 3 The procedure of example 1 is repeated except that the pH ofthe ferricyanide solution is increased to about 13.5 by adding thenecessary amount of potassium hydroxide while the temperature of theoxidizing solution is reduced to about 33F. Similar results areobtained.

The colored surface such as those formed in Examples l-3 may be bleachedwith concentrated, cold sulfuric acid and then re-dyed, such as if thecolor is not correct. This procedure establishes that the coating formedby the process herein is an oxide coating rather than a phosphatecoating.

It will be noted that the present invention provides a process foroxidizing aluminum and for coloring aluminum which is low in cost; doesnot require electrical equipment; and can be applied to small parts suchas pins, bolts and the like as well as large parts as in the prior artanodizing methods. Tl colorant appears to be uniformly distributedthroughout the oxide coating and is adherent to the aluminum part. Theresulting colored aluminum and the aluminum alloys have a deep shade andare more attractive than are those coatings that have heretofore beenobtainable on chemically treated aluminum or aluminum alloy surfaces.

We claim:

1. A process for formingan oxide coating on aluminum and its alloyswhich coating is corrosion and heat resistant, is receptive to coatingand readily dyed, said process comprising the steps ofcleaning thealuminum to the extent necessary and formation of said oxidecoatingusing chemical means consisting of contacting the surface of saidpart with an aqueous alkaline solution of aferricyanide salt having a pHselected from between about 7.5 and 10 or 13 and 14.0, said ferricyanidesalt being present in an amount of at least 0.01 moles per liter ofsolution.

2. The process of claim 1 where the ferricyanide salt is present in anamount of from 0.1 to 0.5 moles per liter of solution.

3. The process of claim 2 where the pH of the ferricyanide solutionvaries between about 7.5 and 10.

4. The process of claim 2 where the pH of the ferricyanide solutionvaries between about 13 and 14.0.

5. The process of claim 2 including the step of contacting the oxidizedaluminum part with a colorant solution.

6. The process of claim 5 where the pH of the ferricyanide solutionvaries between about 7.5 and 10 and the temperature of said solutionvaries between about F and the boiling point of the solution.

7. The process of claim 5 where the pH of the ferricyanide solutionvaries between about 13 and l4.0 and the temperature of the solutionvaries between room temperature and a temperature of about 2F above thefreezing point of the solution.

8. The process of claim 2 where the ferricyanide salt is present in anamount of from 0.2 to 0.3 moles per liter of solution.

9. The process of claim 2 where the ferricyanide solution contains a pHadjustor in an amount sufficient to provide the required pH which pHadjustor is selected from the group consisting of alkali carbonates,alkali or alkaline earth metal hydroxides, alkali tri-basic phosphatesand mixtures thereof.

10. The process of claim 2 where the ferricyanide solution contains aninhibitor for aluminum that prevents darkening of the oxide coating andenhances dyeability.

11. The process of claim 10 where the inhibitor is a tri-basic phosphatecontained in solution in an amount at from 0.01 moles per liter ofsolution to saturation.

12. The process of claim 10 where the inhibitor is a tri-basic phosphatecontained in solution in an amount of from 0.04 to 0.50 moles per literof solution.

13. In a process for coloring a substrate coated either with aluminum oran aluminum alloy including the step of cleaning the aluminum to theextent necessary, oxidizing the aluminum and contacting the aluminumwith a colorant, the improvement comprising formation of said oxidecoating by chemical means consisting of contacting the surface of saidaluminum coating with an aqueous, alkaline solution of a ferricyanidesalt, said solution consisting of at least 0.01 moles per liter of aferricyanide, a pH adjustor in an amount sufficient to provide requiredpH and an inhibitor for aluminum to vprevent darkening of the oxidecoating, said aqueous 15. The process of claim 13 where the inhibitor isa tri-basic phosphate contained in solution in an amount of at least0.01 moles per liter of solution.

16. The process of claim 15 where the aqueous alkaline solution of theferricyanide comprises potassium ferricyanide in an amount of from 0.1to 0.5 moles per liter, trisodium phosphate in an amount of from 0.04 to0.50 per liter of solution, potassium carbonate in an amount sufficientto provide required pH and water to 1 liter.

17. An aqueous solution for oxidizing aluminum and its alloys in aprocess for coloring aluminum, said solutri-basic phosphate.

1. A process for forming an oxide coating on aluminum and its alloys which coating is corrosion and heat resistant, is receptive to coating and readily dyed, said process comprising the steps of cleaning the aluminum to the extent necessary and formation of said oxide coating using chemical means consisting of contacting the surface of said part with an aqueous alkaline solution of a ferricyanide salt having a pH selected from between about 7.5 and 10 or 13 and 14.0, said ferricyanide salt being present in an amount of at least 0.01 moles per liter of solution.
 2. The process of claim 1 where the ferricyanide salt is present in an amount of from 0.1 to 0.5 moles per liter of solution.
 3. The process of claim 2 where the pH of the ferricyanide solution varies between about 7.5 and
 10. 4. The process of claim 2 where the pH of the ferricyanide solution varies between about 13 and 14.0.
 5. The process of claim 2 including the step of contacting the oxidized aluminum part with a colorant solution.
 6. The process of claim 5 where the pH of the ferricyanide solution varies between about 7.5 and 10 and the temperature of said solution varies between about 100*F and the boiling point of the solution.
 7. The process of claim 5 where the pH of the ferricyanide solution varies between about 13 and 14.0 and the temperature of the solution varies between room temperature And a temperature of about 2*F above the freezing point of the solution.
 8. The process of claim 2 where the ferricyanide salt is present in an amount of from 0.2 to 0.3 moles per liter of solution.
 9. The process of claim 2 where the ferricyanide solution contains a pH adjustor in an amount sufficient to provide the required pH which pH adjustor is selected from the group consisting of alkali carbonates, alkali or alkaline earth metal hydroxides, alkali tri-basic phosphates and mixtures thereof.
 10. The process of claim 2 where the ferricyanide solution contains an inhibitor for aluminum that prevents darkening of the oxide coating and enhances dyeability.
 11. The process of claim 10 where the inhibitor is a tri-basic phosphate contained in solution in an amount at from 0.01 moles per liter of solution to saturation.
 12. The process of claim 10 where the inhibitor is a tri-basic phosphate contained in solution in an amount of from 0.04 to 0.50 moles per liter of solution.
 13. IN A PROCESS FOR COLORING A SUBSTRATE COATED EITHER WITH ALUMINUM OR AN ALUMINUM ALLOY INCLUDING THE STEP OF CLEANING THE ALUMINUM TO THE EXTENT NECESSARY, OXIDIZING THE ALUMINUM AND CONTACTING THE ALUMUNUM WITH A COLORANT, THE IMPROVEMENT COMPRISING FORMATION OF SAID OXIDE COATING BY CHEMICAL MENTS CONSISTING OF CONTACTING THE SURFACE OF SAID ALUMINUM COATING WITH AN AQUEOUS, ALKALINE SOLUTION OF A FERRICYANIDE SALT, SAID SOLUTION CONSISTING OF AT LEAST 0.01 MOLES PER LITER OF A FERRICYANIDE, A PH ADJUSTOR IN AN AMOUNT SUFFICIENT TO PROVIDE REQUIRED PH AND AN INHIBITOR FOR ALUMINIUM TO PREVENT DAKENING OF THE OXIDE COATING, SAID AQUEOUS ALKALINE SOLUTION OF FERRICYANIDE HAVING A PH VARYING BETWEEN AVOUT 7.5 AND 13.5.
 14. The process of claim 13 where said ferricyanide salt is an alkali or alkaline earth metal ferricyanide.
 15. The process of claim 13 where the inhibitor is a tri-basic phosphate contained in solution in an amount of at least 0.01 moles per liter of solution.
 16. The process of claim 15 where the aqueous alkaline solution of the ferricyanide comprises potassium ferricyanide in an amount of from 0.1 to 0.5 moles per liter, trisodium phosphate in an amount of from 0.04 to 0.50 per liter of solution, potassium carbonate in an amount sufficient to provide required pH and water to 1 liter.
 17. An aqueous solution for oxidizing aluminum and its alloys in a process for coloring aluminum, said solution comprising a ferricyanide salt in an amount of at least 0.01 moles per liter of solution, a pH adjustor in an amouunt sufficient to provide an alkaline solution having a pH varying between 7.5 and 10 or 13 and 14.0., and an inhibitor to prevent darkening and enhance dyeability of the oxide coating, said inhibitor being present in an amount of at least 0.001 moles per liter.
 18. The solution of claim 17 where the inhibitor is a tri-basic phosphate. 